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  <div class="section" id="how-to-use-attributes">
<h1>How To Use Attributes<a class="headerlink" href="#how-to-use-attributes" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#introduction" id="id1">Introduction</a></p></li>
<li><p><a class="reference internal" href="#attribute" id="id2"><code class="docutils literal notranslate"><span class="pre">Attribute</span></code></a></p></li>
<li><p><a class="reference internal" href="#attributelist" id="id3"><code class="docutils literal notranslate"><span class="pre">AttributeList</span></code></a></p></li>
<li><p><a class="reference internal" href="#attrbuilder" id="id4"><code class="docutils literal notranslate"><span class="pre">AttrBuilder</span></code></a></p></li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id1">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>Attributes in LLVM have changed in some fundamental ways.  It was necessary to
do this to support expanding the attributes to encompass more than a handful of
attributes — e.g. command line options.  The old way of handling attributes
consisted of representing them as a bit mask of values.  This bit mask was
stored in a “list” structure that was reference counted.  The advantage of this
was that attributes could be manipulated with ‘or’s and ‘and’s.  The
disadvantage of this was that there was limited room for expansion, and
virtually no support for attribute-value pairs other than alignment.</p>
<p>In the new scheme, an <code class="docutils literal notranslate"><span class="pre">Attribute</span></code> object represents a single attribute that’s
uniqued.  You use the <code class="docutils literal notranslate"><span class="pre">Attribute::get</span></code> methods to create a new <code class="docutils literal notranslate"><span class="pre">Attribute</span></code>
object.  An attribute can be a single “enum” value (the enum being the
<code class="docutils literal notranslate"><span class="pre">Attribute::AttrKind</span></code> enum), a string representing a target-dependent
attribute, or an attribute-value pair.  Some examples:</p>
<ul class="simple">
<li><p>Target-independent: <code class="docutils literal notranslate"><span class="pre">noinline</span></code>, <code class="docutils literal notranslate"><span class="pre">zext</span></code></p></li>
<li><p>Target-dependent: <code class="docutils literal notranslate"><span class="pre">&quot;no-sse&quot;</span></code>, <code class="docutils literal notranslate"><span class="pre">&quot;thumb2&quot;</span></code></p></li>
<li><p>Attribute-value pair: <code class="docutils literal notranslate"><span class="pre">&quot;cpu&quot;</span> <span class="pre">=</span> <span class="pre">&quot;cortex-a8&quot;</span></code>, <code class="docutils literal notranslate"><span class="pre">align</span> <span class="pre">=</span> <span class="pre">4</span></code></p></li>
</ul>
<p>Note: for an attribute value pair, we expect a target-dependent attribute to
have a string for the value.</p>
</div>
<div class="section" id="attribute">
<h2><a class="toc-backref" href="#id2"><code class="docutils literal notranslate"><span class="pre">Attribute</span></code></a><a class="headerlink" href="#attribute" title="Permalink to this headline">¶</a></h2>
<p>An <code class="docutils literal notranslate"><span class="pre">Attribute</span></code> object is designed to be passed around by value.</p>
<p>Because attributes are no longer represented as a bit mask, you will need to
convert any code which does treat them as a bit mask to use the new query
methods on the Attribute class.</p>
</div>
<div class="section" id="attributelist">
<h2><a class="toc-backref" href="#id3"><code class="docutils literal notranslate"><span class="pre">AttributeList</span></code></a><a class="headerlink" href="#attributelist" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> stores a collection of Attribute objects for each kind of
object that may have an attribute associated with it: the function as a whole,
the return type, or the function’s parameters.  A function’s attributes are at
index <code class="docutils literal notranslate"><span class="pre">AttributeList::FunctionIndex</span></code>; the return type’s attributes are at
index <code class="docutils literal notranslate"><span class="pre">AttributeList::ReturnIndex</span></code>; and the function’s parameters’ attributes
are at indices 1, …, n (where ‘n’ is the number of parameters).  Most methods
on the <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> class take an index parameter.</p>
<p>An <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> is also a uniqued and immutable object.  You create an
<code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> through the <code class="docutils literal notranslate"><span class="pre">AttributeList::get</span></code> methods.  You can add and
remove attributes, which result in the creation of a new <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code>.</p>
<p>An <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> object is designed to be passed around by value.</p>
<p>Note: It is advised that you do <em>not</em> use the <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> “introspection”
methods (e.g. <code class="docutils literal notranslate"><span class="pre">Raw</span></code>, <code class="docutils literal notranslate"><span class="pre">getRawPointer</span></code>, etc.).  These methods break
encapsulation, and may be removed in a future release (i.e. LLVM 4.0).</p>
</div>
<div class="section" id="attrbuilder">
<h2><a class="toc-backref" href="#id4"><code class="docutils literal notranslate"><span class="pre">AttrBuilder</span></code></a><a class="headerlink" href="#attrbuilder" title="Permalink to this headline">¶</a></h2>
<p>Lastly, we have a “builder” class to help create the <code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> object
without having to create several different intermediate uniqued
<code class="docutils literal notranslate"><span class="pre">AttributeList</span></code> objects.  The <code class="docutils literal notranslate"><span class="pre">AttrBuilder</span></code> class allows you to add and
remove attributes at will.  The attributes won’t be uniqued until you call the
appropriate <code class="docutils literal notranslate"><span class="pre">AttributeList::get</span></code> method.</p>
<p>An <code class="docutils literal notranslate"><span class="pre">AttrBuilder</span></code> object is <em>not</em> designed to be passed around by value.  It
should be passed by reference.</p>
<p>Note: It is advised that you do <em>not</em> use the <code class="docutils literal notranslate"><span class="pre">AttrBuilder::addRawValue()</span></code>
method or the <code class="docutils literal notranslate"><span class="pre">AttrBuilder(uint64_t</span> <span class="pre">Val)</span></code> constructor.  These are for
backwards compatibility and may be removed in a future release (i.e. LLVM 4.0).</p>
<p>And that’s basically it! A lot of functionality is hidden behind these classes,
but the interfaces are pretty straight forward.</p>
</div>
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